since people are willing to put numbers and past experiences ...
I too went from longboarding to shortboarding or ... from a 200+ liter board to one smaller
since I was 100 kilos at the time and ready to use 8.0-8.5 m² sails, I purchased a 160 liter / 79 cm wide board
for me that was a perfect start and I was planing within a year, harness, etc
still not in footstraps , but we won't talk about that here 
in today's windsurf world, width is almost more important than volume
there are StarBoard iSonics that are also 80 cm wide and only about 120 liters
liters are for flotation ONLY
early planing comes with width
if you were to go to a JP SLW92 which is 165 liters - you will have NO issues
a BIC Techno is rare, but perhaps better for someone going to such a board - less sensitive deck ...
as Mark SSC has stated - "do NOT rush" - take your time
windsurfing is all about patience anyway

all the best
joe